1. Technical Field
The present invention relates to a virtual image display apparatus that presents video images formed by an image display device to a viewer, and particularly to a virtual image display apparatus suitable for a head-mounted display worn around the head of the viewer.
2. Related Art
A variety of optical systems incorporated in a virtual image display apparatus, such as a head-mounted display (hereinafter also referred to as HMD) worn around the head of a viewer, have been proposed (see Japanese Patent No. 4,926,432).
A virtual image display apparatus, such as an HMD, is desired to achieve further size and weight reduction and an increase in the angle of view without degradation in image quality. Further, when the entire view of the viewer is covered and only video image light is visible to the viewer, the viewer is unable to know what is going on outside and hence feels anxious. Instead, when the viewer is allowed to see the outside superimposed with video images, a new application, such as virtual reality, is created. It is therefore desired to develop a display that does not obstruct the outside view but displays it with video image light superimposed thereon. Further, to improve comfort and the exterior appearance of the viewer who wears an HMD or any other virtual image display apparatus, it is typically desirable that the shape of the apparatus is similar to that of glasses and that video image display devices are disposed sideways and separate from the face of the viewer instead of disposing them above the eyes of the viewer.
Japanese Patent No. 4,926,432 proposes an apparatus including a relay system that focuses an image displayed on an image display device in an intermediate image formation plane and particularly has a plurality of diaphragms that block part of the light flux to reduce the amounts of ghost light and flare. The diaphragms are disposed symmetrically with respect to the optical axis.
To achieve size and weight reduction of a virtual image display apparatus and further increase the angle of view thereof without a decrease in image quality, it is sometimes desired in a prism or any other member that guides video image light, for example, to make full use of a surface having an optical function even a portion close to the edge thereof. When a portion close to the edge is used, however, part of the video image light is likely to be guided in an unintended direction and form ghost light.